Figure 6

Myogenesis in Terebratalia transversa. Overlay of CLSM maximum projection micrograph and light micrograph, anterior faces upward. Scale bars equal 50 μm. (A) Ventral view of an early three-lobed stage with apical lobe (AL), mantle lobe (ML), and pedicle lobe (PL). Discernable are the pedicle musculature (pm), the first anlagen of the serial mantle muscles (double arrowhead), and the setae (se). (B) Ventral view of a slightly older larva with prominent pedicle musculature (pm), anlagen of the setae pouch musculature (arrowheads), and setae (se). (C) Later larval stage, ventral view with pedicle musculature (pm), setae pouch muscles (arrowhead), serial mantle muscles (double arrowhead), and central mantle muscles (empty arrowheads), which are extensions of the dorsal setae muscles. The serial mantle muscles are posteriorly connected to the circular mantle muscle (arrows) and antero-ventrally connected to the U-shaped muscle (empty arrows), which extends from the pedicle muscles to the circular mantle muscle. (D) Lateral view of a later larva with the muscle systems described in C. In addition, the first anlagen of the apical longitudinal musculature (alm), the setae muscles (sm), and the setae (se) are visible. (E) Same stage as in D with prominent pedicle muscles (pm) that are connected to the apical longitudinal muscles (alm). The latter connect to the apical transversal muscle (atm). In addition, the setae pouch muscles (arrowheads), the setae muscles (sm), and the setae (se) are indicated. (F) Fully developed larva, ventral view, with central mantle muscles (empty arrowheads), pedicle muscles (pm), circular mantle muscle (arrows), U-shaped muscle (empty arrows), serial mantle muscles (double arrowheads), setae pouch musculature (arrowheads), setae muscles (sm), apical longitudinal muscles (alm), apical transversal muscle (atm), and setae (se).